70 
COLOUR IN NATURE 
CHAP. 
We have thus considered the common colouring- 
matters of flowers and fruits, and seen that in general 
terms they fall into two groups, the fixed or lipo- 
chrome pigments, occurring in the form of solid 
particles in the cell, and the free or anthocyan pig¬ 
ments occurring in solution in the cell-sap, and 
varying in tint according to its reaction. In flowers, 
further, we frequently find a pure white colour, which 
we have already seen to be an optical colour caused 
by the presence of air-spaces between colourless cells. 
Colours which are not primary colours are caused by 
a superposition of differently coloured elements, or of 
colourless elements on coloured ones. 
Besides the actual bright colour, flowers are often 
conspicuous by the beauty and regularity of their 
markings. Sometimes this is of exceedingly simple 
nature, and bears an obvious relation to the nature 
of the parts. Thus in the snowdrop the petals are 
delicately veined with green, and in the wood-sorrel 
with violet. A veining with green recalls the appear¬ 
ance of some autumnal leaves when the chlorophyll 
seems to linger longest at the sides of the veins of 
the leaf. In the same way the purple veins of wood- 
sorrel (Oxalis ) are quite similar to the reddened veins 
of many leaves in spring, and just as in leaves the 
red colour may occasionally spread over the whole 
leaf, so it is not very uncommon to find in the wood- 
sorrel as a variation that the petals have become 
completely purple. This occurs especially in shady 
places. The markings of many petals are, however, 
of far more complex nature, and bear no obvious 
relation to the structure of the parts. Such are, for 
example, the markings on the labellum in many 
